|
|
Hi All,
Now I'm not a degreed engineer - and I'm certainly not a thermodynamicist - but I've studied the principals involved. I'm starting to wonder if we aren't trying to make this too complicated by looking at it too simply. Mark Steitle <mark.steitle@austin.utexas.edu> wrote:
Ed,
Not to stir the swirl pot, but I thought that it was decided some time back that higher pressures were better than lower pressures. The higher pressures are supposed provide more margin of safety before a boil-over occurs, permitting a thinner ethylene-glycol solution and enabling the engine to tolerate higher temps before experiencing catastrophic boiling. So, I’m wondering where this leaves us. Is it better to run at low (7-8 psi) pressures, or high (28-30psi) pressures? Mark S.
to which Bernie Kerr <jbker@juno.com> responded (in part):
I thought higher pressures were good also because the higher boiling temp, ...
Let's think this through a little more thoroughly, starting with a simpler model - leaving out the radiator for a moment. Instead of dissipating heat, we'll just control the input heat to maintain a stable set of temperatures. The net effect is nearly enough the same as to be negligable.
We have a volume available (e.g. the engine block) "V", occupied by volume "X" of coolant and volume "Y" of a gas (e.g. air/water_vapor) with X being on the order of 30*Y. The exact ratio isn't critical for our low-level discussion.
If we heat the coolant, it will expand, compressing the gas, making the value of "X" larger and "Y" smaller. This will create pressure in the system.
If we keep heating the system, the coolant will continue to expand to whatever limit it can absorb energy without boiling.
As it does so, the volume of "Y" continues to shrink and the pressure goes up. At some point, the pressure gets high enough that it raises the boiling point of the coolant (it may have had to boil some of the coolant into water vapor in order to compress the matter in volume "Y" to that point"). If we stop adding heat at this point (and don't lose any) the system will be stable. If we add more heat, the coolant will either expand or boil more fluid; either of which will raise the pressure.
At some point, adding more heat will cause the pressure to become too much for the system to contain, and something will yield, allowing the pressure to bleed off. We want that to be the pressure cap. What we don't want is for the pressure cap to open and stay open, letting out all the coolant.
What we want from the system is to strike a balance between heat in and heat out (adding the radiator back in) such that the coolant never reaches the point where the pressure exceeds the physical ability of the system to contain it, but that the system temperature is high enough to make for efficient heat transfer.
It really doesn't matter if the volume of "Y" is Zero; as long as there is heat going into the system - and containment - there will be pressure. However, having the value of "Y" greater than zero allows pressure to build more gradually. All that is important is that the value of "Y" isn't so large that a significant amount of coolant has to be turned into vapor before the pressure rises to the point of raising the boiling temperature higher than the system temperature (gas is not an efficient medium for transfer of heat).
So, yes, we want systems *capable* of running at high pressures (18-22#) so that we can run relatively high temps without boiling the coolant. But we don't want to run at the top of the range all the time - hence we need sufficient heat exchange so that there is excess capacity in all operating modes except WOT.
Hope this is some help,
Dale R.
COZY MkIV #1254
|
|